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Abstract:

A switching power source apparatus includes a first arm including first
and second switching elements, a second arm including third and fourth
switching elements, a series circuit connected between a connection point
of the first and second switching elements and a connection point of the
third and fourth switching elements and including a capacitor and a
primary winding, a rectifying-smoothing circuit that rectifies and
smoothes a voltage of a secondary winding and provides an output voltage,
a reactor connected to a connection point of the first and second
switching elements and a DC input end, and a controller that turns on/off
the first and second switching elements alternately and the third and
fourth switching elements alternately and synchronizes the first and
third switching elements with each other and the second and fourth
switching elements with each other.

Claims:

1. A switching power source apparatus comprising: a first arm including a
first switching element and a second switching element connected in
series with the first switching element; a second arm connected in
parallel with the first arm and including a third switching element and a
fourth switching element connected in series with the third switching
element, the first and third switching elements being diagonal to each
other, and the second and fourth switching elements being diagonal to
each other; a first series circuit connected between a connection point
of the first and second switching elements and a connection point of the
third and fourth switching elements and including a first capacitor and a
primary winding of a transformer connected in series with the first
capacitor; a first rectifying-smoothing circuit configured to rectify and
smooth a voltage of a secondary winding of the transformer and provide a
first output voltage; a first reactor connected to a connection point of
the first and second switching elements and one of DC input and output
ends; and a controller that turns on/off the first and second switching
elements alternately, turns on/off the third and fourth switching
elements alternately, synchronizes the first and third switching elements
with each other, and synchronizes the second and fourth switching
elements with each other.

2. The switching power source apparatus of claim 1, wherein the primary
winding of the transformer is connected to a second reactor that is a
leakage inductance between the primary and secondary windings of the
transformer.

3. The switching power source apparatus of claim 1, wherein the
controller synchronizes a signal for turning on the first switching
element with a signal for turning on the third switching element and
synchronizes a signal for turning off the second switching element with a
signal for turning off the fourth switching element.

4. The switching power source apparatus of claim 1, wherein the
controller synchronizes a signal for turning off the first switching
element with a signal for turning off the third switching element and
synchronizes a signal for turning on the second switching element with a
signal for turning on the fourth switching element.

5. The switching power source apparatus of claim 1, wherein: the DC input
end of the first reactor is connected to a DC power source; the DC power
source includes an AC power source and a rectifying circuit; and the
controller carries out power factor correcting control.

6. The switching power source apparatus of claim 1, further comprising a
second capacitor that is connected to both ends of the series circuit of
the third and fourth switching elements and provides a second output
voltage.

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a switching power source apparatus
that is highly efficient and involves a low switching loss.

[0003] 2. Description of Related Art

[0004]FIG. 1 illustrates a switching power source apparatus according to
a related art. This switching power source apparatus includes a step-up
converter and an isolated full-bridge circuit. The step-up converter
includes a DC power source Vin, a reactor L1, switching elements Q1 and
Q2 of MOSFETs, a current detecting resistor R1, a capacitor C1, and a
first controller 101.

[0005] According to a voltage from the capacitor C1 and a voltage from the
current detecting resistor R1, the first controller 101 turns on/off the
switching elements Q1 and Q2 alternately so as to provide, from both ends
of the capacitor C1, a constant output voltage Vo2 that is higher than an
input voltage from the DC power source Vin.

[0006] The isolated full-bridge circuit includes switching elements Q3 to
Q6 that are MOSFETs, current detecting resistors R2 and R3, a primary
winding P1 and secondary windings S1 and S2 of a transformer T, diodes D1
and D2, a rector L2, a capacitor C2, and a second controller 102.

[0007] According to a voltage from the capacitor C2 and voltages from the
current detecting resistors R2 and R3, the second controller 102 turns
on/off the switching elements Q3 and Q6 alternately and the switching
elements Q4 and Q5 alternately so as to provide a constant output voltage
Vo1.

[0008] With this configuration, the switching power source apparatus of
FIG. 1 provides the two output voltages Vo1 and Vo2.

[0010] The switching power source apparatus according to the related art
of FIG. 1 must have six drivers to drive gates of the six switching
elements Q1 to Q6, to complicate the apparatus and increase the cost of
the apparatus.

[0011] Similarly, the related art of Patent Document 1 requires six
drivers 400 as illustrated in FIG. 5 of Patent Document 1, to drive gates
of the three-phase inverter module. This configuration complicates the
three-phase inverter module and increases the cost thereof.

[0012] The present invention provides a switching power source apparatus
that is compact, low-cost, and efficient and realizes zero-voltage
switching.

[0013] According to an aspect of the present invention, the switching
power source apparatus includes a first arm including a first switching
element and a second switching element connected in series with the first
switching element; a second arm connected in parallel with the first arm
and including a third switching element and a fourth switching element
connected in series with the third switching element, the first and third
switching elements being diagonal to each other, the second and fourth
switching elements being diagonal to each other; a first series circuit
connected between a connection point of the first and second switching
elements and a connection point of the third and fourth switching
elements and including a first capacitor and a primary winding of a
transformer connected in series with the first capacitor; a first
rectifying-smoothing circuit that rectifies and smoothes a voltage of a
secondary winding of the transformer and provides a first output voltage;
a first reactor connected to a connection point of the first and second
switching elements and one of DC input and output ends; and a control
unit that turns on/off the first and second switching elements
alternately, turns on/off the third and fourth switching elements
alternately, synchronizes the first and third switching elements with
each other, and synchronizes the second and fourth switching elements
with each other.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic view illustrating a switching power source
apparatus according to a related art;

[0015]FIG. 2 is a schematic view illustrating a switching power source
apparatus according to Embodiment 1 of the present invention;

[0016]FIG. 3 is a schematic view illustrating a control unit in the
switching power source apparatus of FIG. 2;

[0017]FIG. 4 is a timing chart illustrating operation at various
locations in the switching power source apparatus of FIG. 2;

[0018]FIG. 5 is a timing chart illustrating operation at various
locations in the switching power source apparatus of FIG. 2 when the ON
duty of a first converter is greater than that of a second converter in
the switching power source apparatus;

[0019]FIG. 6 is a timing chart illustrating operation at various
locations in the switching power source apparatus of FIG. 2 when the ON
duty of the first converter is smaller than that of the second converter
in the switching power source apparatus;

[0020]FIG. 7 is a timing chart illustrating operation at various
locations in the switching power source apparatus of FIG. 2 when the ON
duty of the first converter is equal to that of the second converter in
the switching power source apparatus;

[0021]FIG. 8 is a schematic view illustrating a switching power source
apparatus according to Embodiment 2 of the present invention;

[0022]FIG. 9 is a schematic view illustrating a switching power source
apparatus according to Embodiment 3 of the present invention;

[0023]FIG. 10 is a timing chart illustrating operation at various
locations in the switching power source apparatus of FIG. 9;

[0024]FIG. 11 is a schematic view illustrating a switching power source
apparatus according to Embodiment 4 of the present invention; and

[0025]FIG. 12 is a timing chart illustrating operation at various
locations in the switching power source apparatus of FIG. 11.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0026] Switching power source apparatuses according to embodiments of the
present invention will be explained in detail with reference to the
drawings.

Embodiment 1

[0027]FIG. 2 is a schematic view illustrating a switching power source
apparatus according to Embodiment 1 of the present invention. This
switching power source apparatus includes a first converter as a step-up
converter and a second converter as a DC-DC converter. The first and
second converters are linked to each other through a transformer T, a
capacitor C2, and a reactor Lr1 with the use of gate pulses.

[0028] The first converter is a synchronous rectification step-up circuit
and includes a reactor L1, a switching element Q1 serving as a main
control switch, a switching element Q2 serving as an auxiliary control
switch (synchronous rectification switch), a capacitor C1, and a first
controller 11. The switching elements Q1 and Q2 are a first arm.

[0029] The second converter is a half-bridge forward converter and
includes a switching element Q3 serving as a main control switch, a
switching element Q4 serving as an auxiliary control switch (synchronous
rectification switch), capacitors C2 and C3, the transformer T, diodes D1
and D2, a reactor L2, and a second controller 12. The switching elements
Q3 and Q4 are a second arm.

[0030] Like the related art of FIG. 1, the switching power source
apparatus according to Embodiment 1 employs a full-bridge configuration
involving the switching elements Q1 to Q4. Operation of the full-bridge
configuration of Embodiment 1, however, quite differs from that of the
related art of FIG. 1. Namely, the full-bridge configuration of
Embodiment 1 operates like a half-bridge configuration with two
half-bridge parts compensating with each other to realize zero-voltage
switching (ZVS).

[0031] In FIG. 2, both ends of a DC power source Vin are connected to a
series circuit including the reactor L1, the switching element Q1, and a
current detecting resistor R1. The switching element Q1 is connected in
series with the switching element Q2.

[0032] Both ends of a series circuit including the switching elements Q1
and Q2 and the current detecting resistor R1 are connected to a series
circuit including the switching elements Q3 and Q4 and a current
detecting resistor R2 as well as the capacitor C1. The capacitor C1
provides an output voltage Vo2. The switching element Q3 is connected to
the switching element Q2 side and the switching element Q4 is connected
to the switching element Q1 side.

[0033] The switching elements Q1 to Q4 are MOSFETs. Connected between
drain and source of the switching element Q1 is a parallel circuit
including a diode Da and a capacitor Ca. Connected between drain and
source of the switching element Q2 is a parallel circuit including a
diode Db and a capacitor Cb. Connected between drain and source of the
switching element Q3 is a parallel circuit including a diode Dc and a
capacitor Cc. Connected between drain and source of the switching element
Q4 is a parallel circuit including a diode Dd and a capacitor Cd. The
diodes Da to Dd are flywheel diodes and may be parasitic diodes of the
switching elements Q1 to Q4, respectively. The capacitors Ca to Cd are
resonant capacitors and may be parasitic capacitors of the switching
elements Q1 to Q4, respectively.

[0034] Connected between a connection point of the switching elements Q1
and Q2 and a connection point of the switching elements Q3 and Q4 is a
series circuit including the capacitor C2, the reactor Lr1, and a primary
winding P1 of the transformer T. The reactor Lr1 may be a leakage
inductance between the primary winding P1 and secondary windings S1 and
S2 of the transformer T.

[0035] The secondary windings S1 and S2 of the transformer T are connected
in series. A first end of the secondary winding S1 is connected to an
anode of the diode D1. A first end of the secondary winding S2 is
connected to an anode of the diode D2. Cathodes of the diodes D1 and D2
are connected to a first end of the reactor L2. A second end of the
reactor L2 is connected to a first end of the capacitor C3. A second end
of the capacitor C3 is connected to a connection point of the secondary
windings S1 and S2. The capacitor C3 provides an output voltage Vo1. The
diodes D1 and D2, reactor L2, and capacitor C3 work as a
rectifying-smoothing circuit.

[0036] A control unit 10 includes the first controller 11, the second
controller 12, and a synchronizer 13. According to a voltage from the
capacitor C1 and a voltage from the current detecting resistor R1, the
first controller 11 turns on/off the switching elements Q1 and Q2
alternately, to carry out step-up control to provide the constant output
voltage Vo2 higher than an input voltage of the DC power source Vin.

[0037] According to a voltage from the capacitor C3 and a voltage from the
current detecting resistor R2, the second controller 12 turns on/off the
switching elements Q3 and Q4 alternately to provide the constant output
voltage Vo1.

[0038] The synchronizer 13 synchronizes the operation timing of the first
controller 11 with the operation timing of the second controller 12. More
precisely, as illustrated in FIG. 4, the synchronizer 13 synchronizes a
rising edge (turn-on timing) of a gate signal Q1g for the switching
element Q1 with a rising edge of a gate signal Q3g for the switching
element Q3 and synchronizes a falling edge (turn-off timing) of a gate
signal Q2g for the switching element Q2 with a falling edge of a gate
signal Q4g for the switching element Q4. Namely, the switching elements
Q1 and Q3 are a diagonal pair in a full-bridge circuit and the switching
elements Q2 and Q4 are another diagonal pair in the full-bridge circuit.

[0039] A midpoint of the first arm made of the switching elements Q1 and
Q2 is connected to a midpoint of the second arm made of the switching
elements Q3 and Q4 through the primary winding P1 of the transformer T,
the reactor Lr1, and the capacitor C2, so that the switching elements Q1
to Q4 may achieve zero-voltage switching (ZVS). The zero-voltage
switching is achievable because, even in an OFF period of the main
control switches Q1 and Q3, a current path of a regenerative current is
always secured through the synchronous rectification switches Q2 and Q4
and diodes Da and Dc.

[0040]FIG. 3 is a schematic view illustrating the control unit
(controller) 10 of the switching power source apparatus according to
Embodiment 1 and FIG. 4 is a timing chart illustrating operation at
various locations in the switching power source apparatus. In the control
unit 10 of FIG. 3, the first controller 11 includes an error amplifier
AMP1, a comparator CMP1, a buffer BUF1, an inverter INV1, and a dead-time
generator 15a. The second controller 12 includes an error amplifier AMP2,
a comparator CMP2, a buffer BUF2, an inverter INV2, and a dead-time
generator 15b. The synchronizer 13 includes a triangular signal generator
130.

[0041] The error amplifier AMP1 amplifies an error voltage between the
output voltage Vo2 from the capacitor C1 and a reference voltage Vref1
and outputs an error amplified signal EAS1 to a non-inverting input
terminal (+) of the comparator CMP1. The error amplifier AMP2 amplifies
an error voltage between the output voltage Vo1 from the capacitor C3 and
a reference voltage Vref2 and outputs an error amplified signal EAS2 to a
non-inverting input terminal (+) of the comparator CMP2.

[0043] If the error amplified signal EAS1 from the error amplifier AMP1 is
equal to or greater than the triangular signal Tria from the triangular
signal generator 130, the comparator CMP1 outputs a high-level signal to
the buffer BUF1 and inverter INV1. If the error amplified signal EAS1 is
lower than the triangular signal Tria, the comparator CMP1 outputs a
low-level signal to the buffer BUF1 and inverter INV1.

[0044] If the error amplified signal EAS2 from the error amplifier AMP2 is
equal to or greater than the triangular signal Tria from the triangular
signal generator 130, the comparator CMP2 outputs a high-level signal to
the buffer BUF2 and inverter INV2. If the error amplified signal EAS2 is
lower than the triangular signal Tria, the comparator CMP2 outputs a
low-level signal to the buffer BUF2 and inverter INV2.

[0045] The buffer BUF1 provides the dead-time generator 15a with the
output of the comparator CMP1. The inverter INV1 inverts the output of
the comparator CMP1 and provides the dead-time generator 15a with the
inverted signal. The dead-time generator 15a delays the signal from the
buffer BUF1 by a predetermined time and generates a gate signal Q1g to be
supplied to a gate of the switching element Q1. The dead-time generator
15a delays the signal from the inverter INV1 by the predetermined time
and generates a gate signal Q2g to be supplied to a gate of the switching
element Q2.

[0046] The buffer BUF2 provides the dead-time generator 15b with the
output of the comparator CMP2. The inverter INV2 inverts the output of
the comparator CMP2 and provides the dead-time generator 15b with the
inverted signal. The dead-time generator 15b delays the signal from the
buffer BUF2 by the predetermined time and generates a gate signal Q3g to
be supplied to a gate of the switching element Q3. The dead-time
generator 15b delays the signal from the inverter INV2 by the
predetermined time and generates a gate signal Q4g to be supplied to a
gate of the switching element Q4.

[0047] Operation of the switching power source apparatus according to
Embodiment 1 will be explained in detail with reference to FIGS. 5 to 7.

[0048] The first converter of the switching power source apparatus has an
ON duty of duty1 and the second converter thereof has an ON duty of
duty2. There are cases such as of (i) duty1>duty2, (ii)
duty1<duty2, and (iii) duty1=duty2 that involve different regenerative
current paths, and therefore, operation of each of the cases will be
explained in detail.

[0049] The following explanation is mainly made in connection with
operation on the primary side of the transformer T. When a switching
element is turned on in a zero-voltage switching mode, the switching
element may involve a plurality of current paths. For example, in a
period T9, the switching element Q1 involves two overlapping current
paths and the switching element Q3 involves one current path, and
therefore, a discharge time of the capacitor Ca of the switching element
Q1 differs from that of the capacitor Cc of the switching element Q3. The
difference in the discharge time, however, is very small. To avoid
complexity, the following explanation assumes that the capacitors Ca and
Cc simultaneously discharge and the diodes Da and Dc simultaneously allow
to cause currents.

(i) Operation Under Condition of duty1>duty2

[0050] Operation of the switching power source apparatus according to
Embodiment 1 when the ON duty duty1 of the first converter of the
apparatus is greater than the ON duty duty2 of the second converter of
the apparatus will be explained with reference to the timing chart of
FIG. 5.

[0051] In a period T1, the switching element Q1 is ON due to the gate
signal Q1g and a current Q1i passes through a path extending along Vin,
L1, Q1, R1, and Vin in a clockwise manner, thereby exciting the reactor
L1. At this time, the switching element Q3 is also ON due to the gate
signal Q3g and a current Q3i passes through a path extending along C1,
Q3, P1, Lr1, C2, Q1, R1, and C1 in a counterclockwise manner, thereby
exciting the reactor Lr1 in this direction. The switching element Q1
allows a sum current of the exciting current of the reactor L1 and the
current Q3i of the switching element Q3.

[0052] A period T2 is a dead-time period. In the period T2, the switching
element Q1 is ON and the current Q1i passes through the path extending
along Vin, L1, Q1, R1, and Vin, thereby exciting the reactor L1. At this
time, the switching element Q3 turns off and the reactor Lr1 starts
discharging the excitation energy thereof, to discharge the capacitor Cd
of the switching element Q4. The reactor Lr1 and the capacitor Cd of the
switching element Q4 resonate to decrease a drain voltage Q4v of the
switching element Q4. At this time, the current Q1i counterclockwise
passes through a path extending along Lr1, C2, Q1, R1, R2, Cd of Q4, P1,
and Lr1.

[0053] A period T3 is a dead-time period. In the period T3, the switching
element Q1 keeps ON and the current Q1i clockwise passes through the path
extending along Vin, L1, Q1, R1, and Vin, thereby exciting the reactor
L1. The reactor Lr1 discharges the excitation energy thereof, to
discharge the capacitor Cd of the switching element Q4 and make the
potential of the switching element Q4 negative. When the negative
potential of the switching element Q4 reaches a forward voltage of the
diode Dd of the switching element Q4, the diode Dd starts to allow a
current counterclockwise passing through a path extending along Lr1, C2,
Q1, R1, R2, Dd of Q4, P1, and Lr1.

[0054] In a period T4, the switching element Q1 is continuously ON and the
current Q1i passes through the path extending along Vin, L1, Q1, R1, and
Vin, thereby exciting the reactor L1. The switching element Q4 turns on
due to the gate signal Q4g and a current Q4i passes through a path
extending along Lr1, C2, Q1, R1, R2, Q4, P1, and Lr1. Namely, the
switching element Q4 achieves zero-voltage switching (ZVS). At this time,
the discharge energy of the reactor Lr1 is regenerated into this path.

[0055] A period T5 is a dead-time period. In the period T5, the switching
element Q1 turns off and the reactor L1 starts to discharge the
excitation energy thereof. At this time, a current passes through a path
extending along Vin, L1, Cb of Q2, C1, and Vin. The path of the discharge
energy of the reactor Lr1 shifts to Lr1, C2, Cb of Q2, C1, R2, Q4, P1,
and Lr1.

[0056] A period T6 is a dead-time period. In the period T6, the capacitor
Cb of the switching element Q2 discharges and the switching element Q2
has negative potential. When the negative potential reaches a forward
voltage of the diode Db of the switching element Q2, the diode Db allows
to cause a current passing through a path extending along Vin, L1, Db of
Q2, C1, and Vin. At this time, the path of the discharge energy of the
reactor Lr1 shifts to Lr1, C2, Cb of Q2, C1, R2, Q4, P1, and Lr1.

[0057] In a period T7, the switching element Q2 turns on and allows to
cause a current Q2i passing through a path extending along Vin, L1, Q2,
C1, and Vin. Namely, the switching element Q2 achieves zero-voltage
switching (ZVS). The current Q2i also passes through a second current
path extending along Lr1, C2, Q2, C1, R2, Q4, P1, and Lr1. Thereafter,
the polarity of the second current path inverts due to resonance between
the reactor Lr1 and the capacitor C2, and therefore, the second current
path shifts to Lr1, P1, Q4, R2, C1, Q2, C2, and Lr1.

[0058] In a period T8, the switching element Q2 is continuously ON and the
current Q2i clockwise passes through the path extending along Vin, L1,
Q2, C1, and Vin. The current Q2i also passes through the second current
path extending along Lr1, C2, Q2, C1, R2, Q4, P1, and Lr1, thereby
exciting the reactor Lr1.

[0059] In a period T9, the switching elements Q2 and Q4 turn off and the
reactor Lr1 starts to discharge the excitation energy thereof and a
current clockwise passes through a path extending along Vin, L1, C2, Lr1,
P1, Cc of Q3, C1, and Vin. Also, a current clockwise passes through a
path extending along Lr1, P1, Cc of Q3, C1, R1, Ca of Q1, C2, and Lr1.

[0060] In a period T10, the capacitors Ca and Cc of the switching elements
Q1 and Q3 discharge to make the potential of the switching elements Q1
and Q3 negative. When the negative potential reaches forward voltages of
the diodes Da and Dc of the switching elements Q1 and Q3, the diodes Da
and Dc allow to cause currents. At this time, a current clockwise passes
through a path extending along Vin, L1, C2, Lr1, Pl, Cc of Q3, C1, and
Vin. Also, a current clockwise passes through a path extending along Lr1,
P1, Cc of Q3, C1, R1, Ca of Q1, C2, and Lr1.

[0061] In a period T11, the switching elements Q1 and Q3 turn on. When the
switching element Q1 turns on, the reactor L1 that has been discharging
the energy thereof starts to be excited with the DC power source Vin. At
this time, a current passes through the path extending along Vin, L1, Q1,
R1, and Vin. Also, a current passes through a second current path
extending along Lr1, P1, Q3, C1, R1, Q1, C2, and Lr1. Namely, the
switching elements Q1 and Q3 achieve zero-voltage switching (ZVS).
Thereafter, the reactor Lr1 and capacitor C2 resonate to invert the
polarity of the second current path, and therefore, the second current
path shifts to Lr1, C2, Q1, R1, C1, Q3, P1, and Lr1.

[0062] (ii) Operation Under Condition of duty1<duty2

[0063] Operation of the switching power source apparatus according to
Embodiment 1 when the ON duty duty1 of the first converter of the
apparatus is smaller than the ON duty duty2 of the second converter of
the apparatus will be explained with reference to the timing chart of
FIG. 6.

[0064] In a period T1, the switching element Q1 is ON due to the gate
signal Q1g and a current Q1i clockwise passes through the path extending
along Vin, L1, Q1, R1, and Vin, thereby exciting the reactor L1. At this
time, the switching element Q3 is also ON due to the gate signal Q3g and
a current Q3i counterclockwise passes through the path extending along
C1, Q3, P1, Lr1, C2, Q1, R1, and C1, thereby exciting the reactor Lr1 in
this direction. The switching element Q1 allows a sum current of the
exciting current of the reactor L1 and the current Q3i of the switching
element Q3.

[0065] In a period T2, the switching element Q1 turns off and the
switching element Q3 is continuously ON. At this time, the discharged
energy of the reactors L1 and Lr1 starts to discharge the capacitor Cb of
the switching element Q2, to cause a current clockwise passing through
the path extending along Vin, L1, Cb of Q2, C1, and Vin and the current
Q3i through the path extending along Lr1, C2, Cb of Q2, Q3, P1, and Lr1.

[0066] In a period T3, the capacitor Cb of the switching element Q2
discharges and the switching element Q2 has negative potential. When the
negative potential reaches the forward voltage of the diode Db of the
switching element Q2, the diode Db allows to cause a current passing
through the path extending along Vin, L1, Db of Q2, C1, and Vin. Also, a
current clockwise passes through a path extending along Lr1, C2, Db of
Q2, Q3, P1, and Lr1.

[0067] In a period T4, the switching element Q2 turns on and allows to
cause a current Q2i passing through the path extending along Vin, L1, Q2,
C1, and Vin. Namely, the switching element Q2 achieves zero-voltage
switching (ZVS). Also, the currents Q2i and Q3i pass through a path
extending along Lr1, C2, Q2, Q3, P1, and Lr1.

[0068] In a period T5, the switching element Q3 turns off, the reactor L1
starts to discharge the excitation energy thereof, and a current
clockwise passes through the path extending along Vin, L1, Q2, C1, and
Vin. At this time, the discharged energy of the reactor Lr1 causes a
current clockwise passing through a path extending along Lr1, C2, Q2, C1,
R2, Cd of Q4, P1, and Lr1, thereby discharging the capacitor Cd of the
switching element Q4.

[0069] In a period T6, the capacitor Cd of the switching element Q4
discharges to make the potential of the switching element Q4 negative.
When the negative potential reaches the forward voltage of the diode Dd
of the switching element Q4, the diode Dd allows to cause a current
passing through the path extending along Vin, L1, Q2, C1, and Vin. Also,
a current passes through a path extending along Lr1, C2, Q2, C1, R2, Dd
of Q4, P1, and Lr1.

[0070] In a period T7, the switching element Q4 turns on and a current
clockwise passes through the path extending along Vin, L1, Q2, C1, and
Vin. Also, a current clockwise passes through a second current path
extending along Lr1, C2, Q2, C1, R2, Q4, P1, and Lr1. Namely, the
switching element Q4 achieves zero-voltage switching (ZVS). Thereafter,
the reactor Lr1 and capacitor C2 resonate to invert the polarity of the
second current path, so that a current counterclockwise passes through
the second current path along Lr1, P1, Q4, R2, C1, Q2, C2, and Lr1.

[0071] In a period T8, the switching elements Q2 and Q4 are continuously
ON and a current clockwise passes through the path extending along Vin,
L1, Q2, C1, and Vin. Also, a current counterclockwise passes through the
second current path extending along Lr1, P1, Q4, R2, C1, Q2, C2, and Lr1,
to excite the reactor Lr1.

[0072] In a period T9, the switching elements Q2 and Q4 turn off, the
reactor Lr1 starts to discharge the excitation energy thereof, and a
current clockwise passes through the path extending along Vin, L1, C2,
Lr1, P1, Cc of Q3, C1, and Vin. Also, a current clockwise passes through
the path extending along Lr1, P1, Cc of Q3, C1, R1, Ca of Q1, C2, and
Lr1.

[0073] In a period T10, the capacitors Ca and Cc of the switching elements
Q1 and Q3 discharge to make the potential of the switching elements Q1
and Q3 negative. When the negative potential reaches the forward voltages
of the diodes Da and Dc of the switching elements Q1 and Q3, the diodes
Da and Dc allows to cause currents. At this time, a current clockwise
passes through the path extending along Vin, L1, C2, Lr1, P1, Cc of Q3,
C1, and Vin. Also, a current clockwise passes through the path extending
along Lr1, P1, Cc of Q3, C1, R1, Ca of Q1, C2, and Lr1.

[0074] In a period T11, the switching elements Q1 and Q3 turn on. When the
switching element Q1 turns on, the reactor L1 that has been discharging
the energy thereof starts to be excited with the DC power source Vin. At
this time, a current clockwise passes through the path extending along
Vin, L1, Q1, R1, and Vin. Also, a current clockwise passes through a
second current path extending along Lr1, P1, Q3, C1, R1, Q1, C2, and Lr1.
Namely, the switching elements Q1 and Q3 achieve zero-voltage switching
(ZVS). Thereafter, the reactor Lr1 and capacitor C2 resonate to invert
the polarity of the second current path, and therefore, a current
counterclockwise passes along Lr1, C2, Q1, R1, C1, Q3, P1, and Lr1.

(iii) Operation Under Condition of duty1=duty2

[0075] Operation of the switching power source apparatus according to
Embodiment 1 when the ON duty duty1 of the first converter of the
apparatus is equal to the ON duty duty2 of the second converter of the
apparatus will be explained with reference to the timing chart of FIG. 7.

[0076] In a period T1, the switching element Q1 is ON due to the gate
signal Q1g and allows to cause a current Q1i clockwise passing through
the route extending along Vin, L1, Q1, R1, and Vin, thereby exciting the
reactor L1. At this time, the switching element Q3 is also ON due to the
gate signal Q3g and allows to cause a current Q3i counterclockwise
passing through the route extending along C1, Q3, P1, Lr1, C2, Q1, R1,
and C1, thereby exciting the reactor Lr1 in this direction. The switching
element Q1 causes a sum current of the exciting current of the reactor L1
and the current Q3i of the switching element Q3.

[0077] In a period T2, the switching elements Q1 and Q3 simultaneously
turn off and the reactors L1 and Lr1 start to discharge their excitation
energy. At this time, a current clockwise passes through the path
extending along Vin, L1, Cb of Q2, C1, and Vin. Also, a current clockwise
passes through a path extending along Lr1, C2, Cb of Q2, C1, R2, Cd of
Q4, P1, and Lr1.

[0078] In a period T3, the capacitors Cb and Cd of the switching elements
Q2 and Q4 discharge to make the potential of the switching elements Q2
and Q4 negative. When the negative potential reaches the forward voltages
of the diodes Db and Dd of the switching elements Q2 and Q4, the diodes
Db and Dd start to cause currents. At this time, a current clockwise
passes through the path extending along Vin, L1, Db of Q2, C1, and Vin.
Also, a current clockwise passes through a path extending along Lr1, C2,
Db of Q2, C1, R2, Dd of Q4, P1, and Lr1.

[0079] In a period T4, the switching elements Q2 and Q4 simultaneously
turn on and a current Q2i clockwise passes through the path extending
along Vin, L1, Q2, C1, and Vin. Also, currents Q2i and Q4i clockwise pass
through a path extending along Lri, C2, Q2, C1, R2, Q4, P1, and Lr1.
Namely, the switching elements Q2 and Q4 achieve zero-voltage switching
(ZVS).

[0080] In a period T5, a current clockwise passes through the path Vin,
L1, Q2, C1, and Vin. Also, a current counterclockwise passes through the
path extending along Lr1, P1, Q4, R2, C1, Q2, C2, and Lr1, to excite the
reactor Lr1.

[0081] In a period T6, the switching elements Q2 and Q4 simultaneously
turn off and the reactor Lr1 starts discharging the excitation energy
thereof, to discharge the capacitors Ca and Cc of the switching elements
Q1 and Q3. At this time, the discharge energy of the reactor L1 changes
its flowing path. A current clockwise passes through the path extending
along Vin, L1, C2, Lr1, P1, Cc of Q3, C1, and Vin. Also, a current
clockwise passes through the path extending along Lr1, P1, Cc of Q3, C1,
R1, Ca of Q1, C2, and Lr1.

[0082] In a period T7, the capacitors Ca and Cc of the switching elements
Q1 and Q3 discharge to make the potential of the switching elements Q1
and Q3 negative. When the negative potential reaches the forward voltages
of the diodes Da and Dc of the switching elements Q1 and Q3, the diodes
Da and Dc allow to cause currents. At this time, a current clockwise
passes through the path extending along Vin, L1, C2, Lr1, P1, Cc of Q3,
C1, and Vin. Also a current clockwise passes through the path extending
along Lr1, P1, Cc of Q3, C1, R1, Ca of Q1, C2, and Lr1.

[0083] In a period T8, the switching elements Q1 and Q3 simultaneously
turn on and the reactor L1 starts to be excited with the DC power source
Vin. At this time, a current clockwise passes through the path extending
along Vin, L1, Q1, R1, and Vin. Also a current clockwise passes through a
second current path extending along Lr1, P1, Q3, C1, R1, Q1, C2, and Lr1.
Namely, the switching elements Q1 and Q3 achieve zero-voltage switching
(ZVS). Thereafter, the reactor Lr1 and capacitor C2 resonate to invert
the polarity of the second current path, and therefore, a current
counterclockwise passes along Lr1, C2, Q1, R1, C1, Q3, P1, and Lr1.

[0084] In this way, the switching power source apparatus according to
Embodiment 1 employs only the four switching elements Q1 to Q4, to
realize compactness and low cost. The midpoints of the first and second
arms are connected to each other through the primary winding P1 of the
transformer T, the capacitor C2, and the reactor Lr1. This configuration
always secures a regenerative current path with the use of the switching
elements Q2 and Q4 serving as synchronous rectification switches and the
diodes Da and Dc even during an OFF period of the switching elements Q1
and Q3 serving as main control switches. As results, the four switching
elements Q1 to Q4 achieve zero-voltage switching to highly improve the
efficiency of the switching power source apparatus. In the switching
power source apparatus according to Embodiment 1, the first and second
converters each carry out synchronous rectification to regenerate the
excitation energy of the reactors through optional paths and realize the
zero-voltage switching.

[0085] According to Embodiment 1, the first controller 11 and second
controller 12 control outputs of their respective converters, to
accurately stabilize the outputs. The drain-source voltage Vds of each of
the switching elements Q1 to Q4, therefore, is clamped with the stable
voltage Vo2 of the capacitor C1.

[0086] According to Embodiment 1, the first converter may be put in a
no-load state and only the second converter may receive load. This
realizes an active clamp circuit with the second converter serving as a
main operation unit. In this case, the first converter carries out
constant-voltage control, and therefore, the drain-source voltage Vds of
each switch is maintained at a constant value, unlike an active clamp
circuit according to a related art.

[0087] In FIG. 4, the simultaneous turning-on of the switching elements Q1
and Q3 and that of the switching elements Q2 and Q4 may each have a
slight deviation in the simultaneity. Without regard to the slight
deviation in the simultaneity, the switching elements Q1 and Q3 (Q2 and
Q4) are able to achieve zero-voltage switching (ZVS) because a
regenerative current, which must have sufficient energy to achieve the
zero-voltage switching, automatically passes through a short path, i.e.,
a low-impedance path.

Embodiment 2

[0088]FIG. 8 is a schematic view illustrating a switching power source
apparatus according to Embodiment 2 of the present invention. Instead of
the DC power source Vin of Embodiment 1 illustrated in FIG. 2, Embodiment
2 of FIG. 8 employs a PFC (power factor correction) circuit involving an
AC power source Vac, a rectifier RC1, and a capacitor C0.

[0089] The AC power source Vac supplies an AC voltage to the rectifier
RC1, which rectifies the AC voltage.

[0090] Both output ends of the rectifier RC1 are connected to the
capacitor C0. A first controller 11 receives a voltage of the capacitor
C0, multiplies the pulsating output voltage of the capacitor C0 by an
output error voltage of a capacitor C1, and according to a result of the
multiplication and a voltage from a current detecting resistor R1,
equalizes an input AC current waveform with an input AC voltage waveform,
thereby correcting a power factor.

[0091] With this configuration, the switching power source apparatus
according to Embodiment 2 corrects a power factor, operates like the
switching power source apparatus according to Embodiment 1, and provides
effects similar to those provided by Embodiment 1.

[0092] According to Embodiment 2, the capacitor C0 and a reactor L1 work
as an LC filter to reduce noise.

Embodiment 3

[0093]FIG. 9 is a schematic view illustrating a switching power source
apparatus according to Embodiment 3 of the present invention. This
switching power source apparatus includes a non-insulated step-down
circuit and an insulated circuit.

[0094] Both ends of a DC power source Vin are connected through a current
detecting resistor R1 to a series circuit including a switching element
Q1 serving as a main control switch and a switching element Q2 serving as
an auxiliary control switch (synchronous rectification switch) and
through a current detecting resistor R2 to a series circuit including a
switching element Q3 serving as a main control switch and a switching
element Q4 serving as an auxiliary control switch (synchronous
rectification switch). Connected between a connection point of the
switching elements Q1 and Q2 and a connection point of the switching
elements Q3 and Q4 is a series circuit including a capacitor C2, a
primary winding P1 of a transformer T, and a reactor Lr1. The secondary
side of the transformer T has the same configuration as that of
Embodiment 1.

[0095] A connection point of the switching elements Q1 and Q2 is connected
to a series circuit including a reactor L1 and a capacitor C1. Both ends
of the capacitor C1 provide an output voltage Vo2.

[0096] According to a voltage from the capacitor C1 and a voltage from the
current detecting resistor R1, a first controller 11a turns on/off the
switching elements Q1 and Q2 alternately, to carry out step-down control
and provide the output voltage Vo2 that is a constant voltage lower than
an input voltage from the DC power source Vin.

[0097] According to a voltage from a capacitor C3 and a voltage from a
current detecting resistor R2, a second controller 12a turns on/off the
switching elements Q3 and Q4 alternately, to provide a constant output
voltage Vo1.

[0098] A synchronizer 13a synchronizes the operation timing of the first
controller 11a with the operation timing of the second controller 12a.
More precisely, as illustrated in FIG. 10, the synchronizer 13a
synchronizes a falling edge of a gate signal Q1g for the switching
element Q1 with a falling edge of a gate signal Q3g for the switching
element Q3 and synchronizes a rising edge of a gate signal Q2g for the
switching element Q2 with a rising edge of a gate signal Q4g for the
switching element Q4. Namely, the switching elements Q1 and Q3 are a
diagonal pair in a full-bridge circuit and the switching elements Q2 and
Q4 are another diagonal pair in the full-bridge circuit.

[0099] A midpoint of a first arm of the switching elements Q1 and Q2 is
connected to a midpoint of a second arm of the switching elements Q3 and
Q4 through the primary winding P1 of the transformer T, the reactor Lr1,
and the capacitor C2, so that the switching elements Q1 to Q4 may achieve
zero-voltage switching (ZVS). The zero-voltage switching is achievable
because, even in an OFF period of the main control switches Q1 and Q3, a
path for passing a regenerative current is always secured through the
synchronous rectification switches Q2 and Q4 and diodes Da and Dc.

[0100] The switching power source apparatus according to Embodiment 3
provides effects similar to those provided by Embodiment 1.

Embodiment 4

[0101]FIG. 11 is a schematic view illustrating a switching power source
apparatus according to Embodiment 4 of the present invention. Embodiment
4 differs from Embodiment 1 of FIG. 2 in that Embodiment 4 additionally
connects a series circuit including switching elements Q5 and Q6 and a
current detecting resistor R3 to both ends of a series circuit including
switching elements Q3 and Q4 and a current detecting resistor R2. The
switching elements Q5 and Q6 are a third arm.

[0102] The switching elements Q5 and Q6 are MOSFETs. Connected between
drain and source of the switching element Q5 is a parallel circuit
including a diode De and a capacitor Ce. Connected between drain and
source of the switching element Q6 is a parallel circuit including a
diode Df and a capacitor Cf. The diodes De and Df are flywheel diodes and
may be parasitic diodes of the switching elements Q5 and Q6,
respectively. The capacitors Ce and Cf are resonant capacitors and may be
parasitic capacitors of the switching elements Q5 and Q6, respectively.

[0103] Connected between a connection point of the switching elements Q1
and Q2 and a connection point of the switching elements Q5 and Q6 is a
series circuit including a capacitor C8, a primary winding P2 of a
transformer Ta, and a reactor Lr2. The reactor Lr2 may be a leakage
inductance between the primary winding P2 and secondary windings S3 and
S4 of the transformer Ta.

[0104] The secondary windings S3 and S4 of the transformer Ta are
connected in series. A first end of the secondary winding S3 is connected
to an anode of a diode D3. A first end of the secondary winding S4 is
connected to an anode of a diode D4. Cathodes of the diodes D3 and D4 are
connected to a first end of a reactor L3. A second end of the reactor L3
is connected to a first end of a capacitor C9. A second end of the
capacitor C9 is connected to a connection point of the secondary windings
S3 and S4. The capacitor C9 provides an output voltage Vo3. The diodes D3
and D4, reactor L3, and capacitor C9 work as a rectifying-smoothing
circuit.

[0105] According to a voltage from the capacitor C9 and a voltage from the
current detecting resistor R3, a third controller 14 turns on/off the
switching elements Q5 and Q6 alternately to provide the constant output
voltage Vo3.

[0106] A synchronizer 13b synchronizes the operation timing of the first,
second, and third controllers 11, 12, and 14 with one another. More
precisely, as illustrated in FIG. 12, the synchronizer 13b synchronizes a
rising edge of a gate signal Q1g for the switching element Q1, a rising
edge of a gate signal Q3g for the switching element Q3, and a rising edge
of a gate signal Q5g for the switching element Q5 with one another and
synchronizes a falling edge of a gate signal Q2g for the switching
element Q2, a falling edge of a gate signal Q4g for the switching element
Q4, and a falling edge of a gate signal Q6g for the switching element Q6
with one another.

[0107] A midpoint of a first arm made of the switching elements Q1 and Q2
is connected to a midpoint of a second arm made of the switching elements
Q3 and Q4 through a primary winding P1 of a transformer T, a reactor Lr1,
and a capacitor C2, so that the switching elements Q1 to Q4 may achieve
zero-voltage switching (ZVS).

[0108] A midpoint of the first arm made of the switching elements Q1 and
Q2 is connected to a midpoint of the third arm made of the switching
elements Q5 and Q6 through the primary winding P2 of the transformer Ta,
the reactor Lr2, and the capacitor C8, so that the switching elements Q1,
Q2, Q5, and Q6 may achieve zero-voltage switching (ZVS).

[0109] In this way, the switching power source apparatus according to
Embodiment 4 outputs three voltages Vo1, Vo2, and Vo3 and provides
effects similar to those provided by Embodiment 1.

[0110] In summary, the switching power source apparatus according to the
present invention employs at least first to fourth switching elements to
realize compactness and low cost. The apparatus connects midpoints of
first and second arms each made of two of the switching elements to each
other through a primary winding of a transformer and a capacitor, to
always secure a path for a regenerative current by use of the second and
fourth switching elements and flywheel diodes even during an OFF period
of the first and third switching elements. With this configuration, the
apparatus realizes zero-voltage switching of the four switching elements
to improve efficiency.

[0111] The present invention is applicable to DC-DC converters, power
factor correction circuits, AC-DC converters, and the like.

[0112] This application claims benefit of priority under 35USC §119
to Japanese Patent Application No. 2011-120114, filed on May 30, 2011,
the entire contents of which are incorporated by reference herein.